We are investigating the application of two photon fluorescent correlation spectroscopy (FCS) to determine the kinetics of protein folding and protein diffusion constants in partially unfolding solutions containing very dilute protein concentrations. T diffusion constants of bovine serum albumin, fatty acid binding protein, carbonic anhydrase, R-phycoerythrin, troponin and adipocyte lipid binding protein have been determined as a function of denaturant (urea or GuHCl) concentration